The bombing capped a violent day that included the death of a 15-year-old Israeli girl from a Hezbollah rocket in a northern Galilee town, and renewed Israeli airstrikes in and around Beirut.

U.N. officials said their observation post near the village of Khiam took a direct hit late yesterday in an Israeli airstrike. Three members of the mission were killed and a fourth was feared dead. Their names and nationalities were not immediately released.

Annan flew to Rome to meet with Secretary of State Condoleezza Rice and diplomats from other nations on the Lebanon crisis. He said he was "shocked and deeply distressed" by what he said was the "apparently deliberate" targeting of the post by Israeli forces.

Annan said he had received personal assurances from Israeli Prime Minister Ehud Olmert that U.N. positions would be spared, and the U.N. commander in South Lebanon, Gen. Alain Pellegrini, had been in repeated contact with Israeli officers to ensure the post's protection.

U.S. officials labeled the attack "a terrible tragedy" and said they were told by the Israelis that it was an accident.

Israel said it "regrets the tragic death" of the observers and that the incident was under investigation.

Israel's ambassador to the United States reacted sharply to Annan's allegation that the strike was deliberate. "I think this kind of rhetoric is deplorable, it's outrageous, and I hope he will apologize," Ambassador Daniel Ayalon said on CNN's The Situation Room.

In a televised address late yesterday, Hezbollah's leader, Sheik Hassan Nasrallah said his organization would not submit to "humiliating" conditions imposed by the international community for a cease-fire and threatened attacks even deeper into Israel.

Referring to a "new period" in the 15-day-old conflict, he said Hezbollah would strike beyond the port of Haifa, Israel's third-largest city, where rockets have been falling by the dozens.

"We will choose the time when we will move beyond - beyond Haifa," Nasrallah said.

The U.N. deaths come as Western nations were set to meet today in Rome to discuss a possible cease-fire, response to the growing humanitarian crisis in Lebanon and a possible international peacekeeping force.

The Bush administration's stance that Israel should be allowed to deal a more decisive blow to Hezbollah before any cease-fire did not budge. In Jerusalem, Rice stood by Olmert as he pledged to "carry on the fight" against Hezbollah. Rice visited the West Bank city of Ramallah, and again turned aside calls from the Lebanese for an immediate cease-fire. She said an "enduring" peace was more important and possible only with the disarming of Hezbollah.

"It is time for a new Middle East," she said, with Olmert at her side. "And to those who do not want a new Middle East, we say we will prevail, they will not."

"Israel is determined to carry on the fight against Hezbollah," Olmert said. "We will stop them. We will not hesitate to take the most severe measures against those who are aiming thousands of missiles - and missiles against innocent civilians for one purpose - to kill them."

Later, Rice flew to Rome to join European and Arab diplomats in hastily called crisis talks.

Henry A. Crumpton, the State Department's coordinator for counterterrorism, said yesterday that he believed the Israeli response was "in some ways just beginning," noting that Israel's military has made only limited progress in degrading Hezbollah capabilities.

Ground fighting continued, with Israeli armor and infantry battling guerrillas for control of the Lebanese village of Bint Jbeil. By the end of the day, field commanders said they had seized the town, the largest in the border region and considered by Israel to be a Hezbollah headquarters. The Israeli military said 20 to 30 Hezbollah gunmen were killed in the fighting.

Israeli forces were expanding their attack to other stretches near the border. Army convoys could be seen hauling tanks and bulldozers to areas that have not yet seen large concentrations of forces. Israeli artillery shelled Lebanese villages north of Metulla, at the northern tip of the upper Galilee. Expecting Hezbollah reprisals, Metulla officials told residents to leave or seek refuge in bomb shelters.

Israeli Defense Minister Amir Peretz said his forces were continuing to carve out a "security strip" that he said Israel would maintain as a no-go zone until the eventual deployment of multinational forces - something that is far from decided.